Method for alignment, process tool and method for wafer-level alignment

ABSTRACT

Various embodiments of the present application are directed towards a method for workpiece-level alignment with low alignment error and high throughput. In some embodiments, the method comprises aligning a first alignment mark on a first workpiece to a field of view (FOV) of an imaging device based on feedback from the imaging device, and further aligning a second alignment mark on a second workpiece to the first alignment mark based on feedback from the imaging device. The second workpiece is outside the FOV during the aligning of the first alignment mark. The aligning of the second alignment mark is performed without moving the first alignment mark out of the FOV. Further, the imaging device views the second alignment mark, and further views the first alignment mark through the second workpiece, during the aligning of the second alignment mark. The imaging device may, for example, perform imaging with reflected infrared radiation.

REFERENCE TO RELATED APPLICATION

This Application is a Continuation of U.S. application Ser. No. 16/015,507, filed on Jun. 22, 2018, the contents of which are incorporated by reference in their entirety.

BACKGROUND

Wafer-to-wafer (W2W) bonding is a process by which two wafers are bonded together. W2W bonding is often employed during the formation of, among other things, three-dimensional integrated circuits (3DICs), microelectromechanical systems (MEMS) devices, and silicon-on-insulator (SOI) wafers. Wafer-level alignment is a process by which two wafers are aligned and is performed before and/or during W2W bonding to ensure proper alignment. Alignment error may be quantified as the sum of translation error, tool stability error, rotational error, run-out error, and measurement error. Further, translation error, tool stability error, and rotation error are typically the largest contributors to alignment error.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A and 1B illustrate cross-sectional schematic diagrams of various embodiments of a bond tool with high alignment accuracy and high throughput.

FIGS. 2A and 2B illustrate cross-sectional schematic diagrams of various alternative embodiments of the bond tool of FIGS. 1A and 1B in which the bond tool is configured for alignment at two or more locations.

FIGS. 3A-3D illustrate top layouts of various embodiments of a workpiece for use in any one of FIGS. 1A, 1B, 2A, and 2B.

FIGS. 4A and 4B illustrate top layouts of various embodiments of a workpiece stage for use in any one of FIGS. 1A, 1B, 2A, and 2B.

FIG. 5 illustrates a cross-sectional view of some embodiments of a three-dimensional integrated circuit (3DIC) chip comprising workpieces aligned by the bond tool in any one of FIGS. 1A, 1B, 2A, and 2B.

FIGS. 6A-6C illustrate cross-sectional schematic diagrams of some embodiments of a bond tool at various stages of a method for workpiece-level alignment and bonding.

FIGS. 7A-7C illustrate cross-sectional schematic diagrams of some alternative embodiments of the bond tool of FIGS. 6A-6C at various stages of the method of FIGS. 6A-6C.

FIG. 8 illustrates a block diagram of some embodiments of the method of FIGS. 6A-6C and 7A-7C.

FIG. 9 illustrates alignment between alignment marks in FIGS. 6A-6C and 7A-7C according to some embodiments.

DETAILED DESCRIPTION

The present disclosure provides many different embodiments, or examples, for implementing different features of this disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

According to a method for wafer-level alignment, a top imaging device and a bottom imaging device are focused on a calibration mark. A bottom wafer is moved laterally by a bottom wafer stage until an alignment mark on the bottom wafer is recognized by the top imaging device. The aligned position of the bottom wafer is stored and the bottom wafer is moved out of a field of view of the top imaging device. A top wafer is moved laterally by a top wafer stage until an alignment mark on the top wafer is recognized by the bottom imaging device. The bottom wafer is moved laterally by the bottom wafer stage to the stored position. The top and bottom wafers may then be bonded together.

A challenge with the method for wafer-level alignment is that it has high alignment error and low throughput. The top and bottom imaging devices use visible light, whereby the top and bottom imaging devices are unable to see through the top and bottom wafers. As a result, the alignment mark on the bottom wafer and the alignment on the top wafer cannot be recognized simultaneously. Further, the bottom wafer is moved into its aligned position from memory. In a sense, the bottom wafer is moved into its aligned position blindly since it's done from memory and the top wafer obstructs the top imaging device's view of the bottom wafer. Moving the bottom wafer from its aligned position and then back again from memory introduces a high degree of mechanical or tool stability error and slows throughput.

Another method for wafer-level alignment uses total internal reflection and infrared radiation to align the top wafer and the bottom wafer in a vacuum chamber. The infrared radiation is emitted towards reflectors in a top wafer stage. The infrared radiation undergoes TIR at the reflectors and is reflected downward through the top wafer and the bottom wafer to infrared imaging devices underlying the bottom wafer. A bottom wafer stage moves the bottom wafer based on feedback from the infrared imaging devices to align alignment marks on the bottom wafer respectively to alignment marks on the top wafer. However, a challenge with the method is that it has low throughput.

Various embodiments of the present application are directed towards a method for workpiece-level alignment with low alignment error (i.e., high accuracy) and high throughput, as well as a process tool for performing the method. A workpiece may, for example, be or comprise a wafer. In some embodiments, the method comprises aligning a first alignment mark on a first workpiece to a field of view (FOV) of an imaging device based on feedback from the imaging device, and further comprises aligning a second alignment mark on a second workpiece to the first alignment mark based on feedback from the imaging device. The second workpiece is outside the FOV during the aligning of the first alignment mark. Further, the aligning of the first alignment mark may, for example, be performed by: 1) moving the FOV while the first workpiece remains stationary; 2) moving the first workpiece while the FOV remains stationary; or 3) some other process. The aligning of the second alignment mark is performed without moving the first alignment mark out of the FOV. Further, the imaging device views the second alignment mark, and further views the first alignment mark through the second workpiece, during the aligning of the second alignment mark. In some embodiments, the imaging device uses infrared radiation to view the first and second alignment marks.

By using an imaging device capable of viewing the first alignment mark through the second workpiece during the aligning of the second alignment mark, the first and second alignment marks may be concurrently viewed and alignment between the first and second alignment marks may be based on real time feedback on the locations of the first and second alignment marks. As a result, the number of mechanical movements to perform alignment is low and alignment error is low (i.e., alignment accuracy is high). The low number of mechanical movements, in turn, increases throughput. Further, by using an imaging device capable of viewing the first alignment mark through the second workpiece during the aligning of the second alignment mark, alignment may be performed without multiple imaging devices (e.g., top and bottom imaging devices). This, in turn, reduces costs and system complexity.

With reference to FIG. 1A, a cross-sectional schematic diagram 100A of some embodiments of a bond tool with high alignment accuracy (i.e., low alignment error) and high throughput is provided. The bond tool is configured to align a lower workpiece 102 and an upper workpiece 104 to each other, and to further bond the lower and upper workpieces 102, 104 together. In some embodiments, the aligning and/or the bonding is/are performed at atmospheric pressure or some other pressure. The lower workpiece 102 comprises a lower alignment mark 106 a, and the upper workpiece 104 comprises an upper alignment mark 108 a. In some embodiments, the lower alignment mark 106 a and/or the upper alignment mark 108 a is/are cross shaped or some other shape. Further, in some embodiments, the lower and upper alignment marks 106 a, 108 a are copper, some other metal, or some other material.

A lower workpiece stage 110 is configured to support and move the lower workpiece 102. Further, an upper workpiece stage 112 is over the lower workpiece stage 110 and is configured to support and move the upper workpiece 104. In some embodiments, the lower and upper workpiece stages 110, 112 are associated with or comprise actuators (not shown) to implement the movement of the lower and upper workpieces 102, 104.

An imaging device 114 a is over the upper workpiece stage 112 and is oriented so a FOV of the imaging device 114 a is directed towards the lower workpiece stage 110. The imaging device 114 a is configured to perform imaging using radiation 116 capable of passing through the lower and upper workpieces 102, 104, but not through the lower and upper alignment marks 106 a, 108 a. This, in turn, allows the imaging device 114 a to view and/or image the lower alignment mark 106 a through the upper workpiece 104. The radiation 116 may, for example, be or comprise infrared radiation or some other radiation. The infrared radiation may, for example, be or comprise radiation with wavelengths between about 700 nanometers to about 1 millimeter. In some embodiments, the radiation 116 is incapable of passing through the upper workpiece stage 112. In such embodiments, the upper workpiece stage 112 defines an upper alignment opening 118 a extending through the upper workpiece stage 112 to allow the radiation 116 to pass through the upper workpiece stage 112. Further, the upper workpiece 104 is placed on the upper workpiece stage 112 so the upper alignment mark 108 a is aligned with the upper alignment opening 118 a during use of the bond tool. In some embodiments, the radiation 116 is also incapable of passing through the lower workpiece stage 110.

The imaging device 114 a comprises an image sensor 120 a and, in some embodiments, a lens 122 a. The image sensor 120 a is configured to sense radiation, such as, for example, radiation reflected off the lower and upper alignment marks 106 a, 108 a. The image sensor 120 a may, for example, be a charge-coupled device (CCD) image sensor, a CMOS image sensor, or some other image sensor. The lens 122 a is configured to focus radiation on the image sensor 120 a. By focusing radiation on the image sensor 120 a, the lens 122 a enables finer alignment between the lower and upper workpieces 102, 104. In some embodiments, the lens 122 a is also configured to manipulate (e.g., move, reshape, etc.) the FOV of the imaging device 114 a. For example, the lens 122 a may be a zoom lens, whereby the lens 122 a may increase or decrease the focal length of the imaging device 114 a. In some embodiments, the imaging device 114 a is associated with or comprises actuators enabling electronic control over parameters (e.g., focal length) of the lens 122 a. Further, in some embodiments, the imaging device 114 a is associated with or comprises actuators enabling movement of the imaging device 114a.

A controller 124 is configured to control components of the bond tool (e.g., the upper workpiece stage 112) to laterally move the lower and upper workpieces 102, 104 into alignment based on feedback from the imaging device 114 a. For example, the upper workpiece stage 112 may be controlled to laterally move the upper workpiece stage 112 and, hence, the upper workpiece 104 outside the FOV of the imaging device 114 a, the lower alignment mark 106 a may then be aligned to the FOV, and the upper workpiece stage 112 may be controlled to laterally move the upper workpiece 104 until the upper alignment mark 108 a is aligned to the lower alignment mark 106 a in the FOV. The alignment of the lower alignment mark 106 a to the FOV may, for example, be performed by: 1) controlling actuators to laterally move the imaging device 114 a and, hence, the FOV while the lower workpiece 102 remains stationary; 2) controlling the lower workpiece stage 110 to move the lower workpiece 102 while the FOV and, hence, the imaging device 114 a remain stationary; 3) controlling actuators and/or the lens 122 a to optically move and/or manipulate the FOV while the imaging device 114 a and the lower workpiece 102 remain stationary; or 4) some other process. The alignment of the upper alignment mark 108 a may, for example, be performed while the FOV and the lens 122 a are fixed.

Because the imaging device 114 a may view the lower alignment mark 106 a through the upper workpiece 104, the lower and upper alignment marks 106 a, 108 a may be concurrently viewed and alignment between the lower and upper alignment marks 106 a, 108 a may be based on real time feedback on locations of the lower and upper alignment marks 106 a, 108 a. As a result, alignment accuracy is high (i.e., alignment error is low) and the number of mechanical movements to perform alignment is low. The low number of mechanical movements, in turn, leads to high throughput and a low cost of ownership. For example, alignment and bonding may be performed in less than about 160, 140, or 120 seconds.

The controller 124 is further configured to control the lower workpiece stage 110 and/or the upper workpiece stage 112 to vertically move the lower and upper workpieces 102, 104 into contact for bonding after alignment. The bonding may, for example, be performed by fusion bonding, hybrid bonding, or some other bonding.

Functionality of the controller 124 may, for example, be implemented in hardware, software, or a combination of the two. In some embodiments, the controller 124 is or comprises electronic data storage, and further comprises one or more electronic processors configured to execute processor executable instructions that are on the electronic data storage and that implement some or all functionality of the controller 124. In some embodiments, the controller 124 is or comprises an application-specific integrated circuit (ASIC) or some other application-specific hardware implementing some or all functionality of the controller 124.

In some embodiments, a radiation source 126 a is over the upper workpiece stage 112. In some embodiments, the radiation source 126 a is integrated with the imaging device 114 a. In other embodiments, the radiation source 126 a is separate from, but adjacent to, the imaging device 114 a. The radiation source 126 a is configured to emit radiation of the type sensed by the imaging device 114 a towards the lower workpiece stage 110 so as to illuminate the FOV of the imaging device 114 a. For example, where the imaging device 114 a senses infrared radiation, the radiation source 126 a may be configured to emit infrared radiation towards the lower workpiece stage 110. In some embodiments, the imaging device 114 a senses radiation emitted from the radiation source 126 a that is reflected back to the imaging device 114, and the lower and upper alignment marks 106 a, 108 a are detected by the presence of reflected radiation since the emitted radiation may reflect off the lower and upper alignment marks 106 a, 108 a.

In some embodiments, each of the lower and upper workpieces 102, 104 is or comprises a silicon wafer or some other wafer. In some embodiments, the lower workpiece 102 is or comprises a silicon wafer or some other wafer, and the upper workpiece 104 comprises a substrate (e.g., a silicon wafer or some other wafer) and an integrated circuit (IC) structure on the substrate, or vice versa. In some embodiments, each of the lower and upper workpieces 102, 104 comprises a substrate (e.g., a silicon wafer or some other wafer) and an IC structure on the substrate. An IC structure comprises semiconductor devices, and further comprises a stack of dielectric and conductive layers stacked on the semiconductor devices. The dielectric and conductive layers may, for example, define an interconnect structure.

In some embodiments, the bond tool is employed for microelectromechanical systems (MEMS) bonding, memory stacking, logic stacking, complementary metal-oxide-semiconductor (CMOS) image sensor bonding, or some other stacking or bonding. For example, where the bond tool is employed for logic stacking, the lower and upper workpieces 102, 104 may be CMOS wafers. As another example, where the bond tool is employed for MEMS bonding, the lower and upper workpieces 102, 104 may, for example, respectively be a CMOS wafer and a bulk silicon wafer.

With reference to FIG. 1B, a cross-sectional schematic diagram 100B of some alternative embodiments of the bond tool of FIG. 1A is provided in which the radiation source 126 a is below the lower workpiece stage 110. In some embodiments, the imaging device 114 a of FIG. 1B senses radiation transmitted through the lower and upper workpieces 102, 104, and the lower and upper alignment marks 106 a, 108 a may be detected by the absence of transmitted radiation since the radiation emitted by the radiation source 126 a may not pass through the lower and upper alignment marks 106 a, 108 a. In some embodiments, radiation emitted by the radiation source 126 a is incapable of passing through the lower workpiece stage 110. In such embodiments, the lower workpiece stage 110 defines a lower alignment opening 128 a extending through the lower workpiece stage 110 to allow the emitted radiation to pass through the lower workpiece stage 110 and to illuminate the FOV of the imaging device 114 a. Further, the lower workpiece 102 is placed on the lower workpiece stage 110 so the lower alignment mark 106 a is aligned with the lower alignment opening 128 a during use of the bond tool.

With reference to FIG. 2A, a cross-sectional schematic diagram 200A of some alternative embodiments of the bond tool of FIG. 1A is provided in which the bond tool is configured for alignment at two or more locations. The lower workpiece 102 comprises the lower alignment mark 106 a, and further comprises a second lower alignment mark 106 b. The upper workpiece 104 comprises the upper alignment mark 108 a, and further comprises a second upper alignment mark 108 b. In some embodiments, the second lower alignment mark 106 b and/or the second upper alignment mark 108 b is/are cross shaped or some other shape. In some embodiments, the second lower alignment mark 106 b and/or the second upper alignment mark 108 b is/are metal or some other material. In some embodiments, the second lower alignment mark 106 b and the lower alignment mark 106 a have the same shape, and/or the second upper alignment mark 108 b and the upper alignment mark 108 a have the same shape.

In addition to the imaging device 114 a, a second imaging device 114 b is over the upper workpiece stage 112 and is oriented so a FOV of the second imaging device 114 b is directed towards the lower workpiece stage 110. The second imaging device 114 b is configured to perform imaging using the radiation 116 capable of passing through the upper workpiece 104 to allow the second imaging device 114 b to view and/or image the second lower alignment mark 106 b through the upper workpiece 104. In some embodiment, the second imaging device 114 b is as the imaging device 114 a is described with regard to FIG. 1A.

In some embodiments, the radiation 116 is incapable of passing through the upper workpiece stage 112. In such embodiments, the upper workpiece stage 112 defines the upper alignment opening 118 a, and further defines a second upper alignment opening 118 b. The upper alignment opening 118 a and the second upper alignment opening 118 b extend through the upper workpiece stage 112 to allow the radiation 116 to pass through the upper workpiece stage 112. During use of the bond tool, the upper workpiece 104 is placed on the upper workpiece stage 112 so the upper alignment mark 108 a and the second upper alignment mark 108 b are respectively aligned with the upper alignment opening 118 a and the second upper alignment opening 118 b.

The second imaging device 114 b comprises a second image sensor 120 b and, in some embodiments, a second lens 122 b. The second image sensor 120 b is configured to sense radiation, such as, for example, radiation reflected off the second lower and upper alignment marks 106 b, 108 b. The second image sensor 120 b may, for example, be a CCD image sensor, a CMOS image sensor, or some other image sensor. The second lens 122 b is configured to focus radiation on the second image sensor 120 b. The second lens 122 b may, for example, be as the lens 122 a is described with regard to FIG. 1A.

The controller 124 is configured to control components of the bond tool to laterally move the lower and upper workpieces 102, 104 into alignment based on feedback from both the imaging device 114 a and the second imaging device 114 b. For example, the upper workpiece stage 112 may be controlled to laterally move the upper workpiece stage 112 outside the FOVs of the imaging device 114 a and the second imaging device 114 b. Then the lower alignment mark 106 a and the second lower alignment mark 106 b may, for example, be aligned respectively to the FOVs. Then the upper workpiece stage 112 may, for example, be controlled to laterally move the upper workpiece 104 until the upper alignment mark 108 a and the second upper alignment mark 108 b are respectively aligned to the lower alignment mark 106 a and the second lower alignment mark 106 b in the FOVs. The alignment of the lower alignment mark 106 a and the second lower alignment mark 106 b respectively to the FOVs may, for example, be performed by: 1) controlling actuators to laterally move the imaging device 114 a and the second imaging device 114 b (and hence the FOVs) while the lower workpiece 102 remains stationary; 2) controlling the lower workpiece stage 110 to move the lower workpiece 102 while the FOVs (and hence the first imaging device 114 a and the second imaging device 114 b) remain stationary; 3) controlling actuators (e.g., actuators associated with or part of the lens 122 a and the second lens 122 b) to optically move and/or manipulate the FOVs while the imaging device 114 a, the second imaging device 114 b, and the lower workpiece 102 remain stationary; or 4) some other process. The alignment of the upper alignment mark 108 a and the second upper alignment mark 108 b may, for example, be performed while the FOV and the lens 122 a are fixed. The controller 124 is further configured to control the lower workpiece stage 110 and/or the upper workpiece stage 112 to vertically move the lower and upper workpieces 102, 104 into contact for bonding after alignment.

Because two or more locations are used for alignment, the lower and upper workpieces 102, 104 may be rotationally aligned. Because the imaging device 114 a and the second imaging device 114 b may respectively view the lower alignment mark 106 a and the second lower alignment mark 106 b through the upper workpiece 104, the lower and upper alignment marks 106 a, 108 a and the second lower and upper alignment marks 106 b, 108 b may be concurrently viewed. As such, alignment between the lower and upper workpieces 102, 104 may be based on real time feedback on locations of the lower and upper alignment marks 106 a, 108 a and locations of the second lower and upper alignment marks 106 b, 108 b. As a result, alignment accuracy is high and the number of mechanical movements to perform alignment is low. The low number of mechanical movements, in turn, leads to high throughput and a low cost of ownership.

In some embodiments, in addition to the radiation source 126 a, a second radiation source 126 b is over the upper workpiece stage 112. The second radiation source 126 b is configured to emit radiation of the type sensed by the second imaging device 114 b towards the lower workpiece stage 110 so as to illuminate the FOV of the second imaging device 114 b. For example, where the second imaging device 114 b senses infrared radiation, the second radiation source 126 b may be configured to emit infrared radiation towards the lower workpiece stage 110.

With reference to FIG. 2B, a cross-sectional schematic diagram 200B of some alternative embodiments of the bond tool of FIG. 1B is provided in which alignment is performed at two or more locations. The cross-sectional schematic diagram 200B of FIG. 2B is mostly as the cross-sectional schematic diagram 200A of FIG. 2A is described, except that: 1) the radiation source 126 a and the second radiation source 126 b are below the lower workpiece stage 110; and 2) the lower workpiece stage 110 has a different configuration in some embodiments.

Radiation emitted by the radiation source 126 a and the second radiation source 126 b is incapable of passing through the lower workpiece stage 110 in some embodiments. In such embodiments, the lower workpiece stage 110 defines the lower alignment opening 128 a, and further defines a second lower alignment opening 128 b. The lower alignment opening 128 a and the second lower alignment opening 128 b extend through the lower workpiece stage 110 to allow the emitted radiation to pass through the lower workpiece stage 110 and illuminate the FOVs of the imaging device 114 a and the second imaging device 114 b. During use of the bond tool, the lower workpiece 102 is placed on the lower workpiece stage 110 so the lower alignment mark 106 a and the second lower alignment mark 106 b are respectively aligned with the lower alignment opening 128 a and the second lower alignment opening 128 b.

With reference to FIG. 3A, a top layout 300A of some embodiments of a workpiece 302 for use in any one of FIGS. 1A, 1B, 2A, and 2B is provided. In some embodiments, the workpiece 302 is representative of both the lower and upper workpieces 102, 104 in any one of FIGS. 1A, 1B, 2A, and 2B. In some embodiments, the workpiece 302 is representative of the lower workpiece 102, but not the upper workpiece 104, in any one of FIGS. 1A, 1B, 2A, and 2B. In some embodiments, the workpiece 302 is representative of the upper workpiece 104, but not the lower workpiece 102, in any one of FIGS. 1A, 1B, 2A, and 2B.

In embodiments in which the workpiece 302 is representative of the lower workpiece 102 and/or the upper workpiece 104 in FIG. 1A and/or FIG. 1B, the lower workpiece 102 and/or the upper workpiece 104 may, for example, be taken along line A. In embodiments in which the workpiece 302 is representative of the lower workpiece 102 and/or the upper workpiece 104 in FIG. 2A and/or FIG. 2B, the lower workpiece 102 and/or the upper workpiece 104 may, for example, be taken along line B.

The workpiece 302 comprises a plurality of exposure regions 304 and a plurality of alignment marks 306. For ease of illustration, only some of the exposure regions 304 are labeled 304, and only some of the alignment marks 306 are labeled 306. The exposure regions 304 are arranged in grid pattern across the workpiece 302. Each of the exposure regions 304 is a region of the workpiece 302 that a photolithography stepper or scanner stopped at and individually exposed to patterned radiation while stepping across the workpiece 302. The alignment marks 306 facilitate alignment of the workpiece 302 to another workpiece, a reticle, or some other structure. In some embodiments, the alignment marks 306 correspond to the exposure regions 304 with a one-to-one correspondence. Further, in some embodiments, the alignment marks 306 are cross shaped, some other shapes, or any combination of the foregoing.

In some embodiments, the workpiece 302 further comprises a plurality of IC die regions 308 arranged in a grid pattern across the workpiece 302. For ease of illustration, only some of the IC die regions 308 are labeled 308. In some embodiments, the IC die regions 308 correspond to the exposure regions 304 with a one-to-one correspondence. In some embodiments, each of the IC die regions 308 individually has a CMOS image sensor, a MEMS device, a CMOS logic device, a memory device, or some other device.

With reference to FIG. 3B, a top view 300B of some alternative embodiments of the workpiece 302 of FIG. 3A is provided in which the alignment marks 306 are laterally offset from centers of corresponding ones of the exposure regions 304.

With reference to FIG. 3C, a top view 300C of some alternative embodiments of the workpiece 302 of FIG. 3A is provided in which the IC die regions 308 correspond to the exposure regions 304 with a one-to-many correspondence. For example, each of the IC die regions 308 may correspond to four of the exposure regions 304.

With reference to FIG. 3D, a top view 300D of some alternative embodiments of the workpiece 302 of FIG. 3A is provided in which the alignment marks 306 correspond to the exposure regions 304 with a many-to-one correspondence. For example, there may be two or more alignment marks per exposure area.

With reference to FIG. 4A, a top layout 400A of some embodiments of a workpiece stage 402 for use in any one of FIGS. 1A, 1B, 2A, and 2B is provided. In some embodiments, the workpiece stage 402 is representative of both the lower and upper workpiece stages 110, 112 in FIG. 1B and/or FIG. 2B. In some embodiments, the workpiece stage 402 is representative of the upper workpiece stage 112, but not the lower workpiece stage 110, in any one of FIGS. 1A, 1B, 2A, and 2B. In some embodiments, the workpiece stage 402 is or is representative of the lower workpiece stage 110, but not the upper workpiece stage 112, in FIG. 1B and/or FIG. 2B.

In embodiments in which the workpiece stage 402 is representative of the lower workpiece stage 110 and/or the upper workpiece stage 112 in FIG. 1A and/or FIG. 1B, the lower workpiece stage 110 and/or the upper workpiece stage 112 may, for example, be taken along line A. In embodiments in which the workpiece stage 402 is representative of the lower workpiece stage 110 and/or the upper workpiece stage 112 in FIG. 2A and/or FIG. 2B, the lower workpiece stage 110 and/or the upper workpiece stage 112 may, for example, be taken along line B.

The workpiece stage 402 is configured to support and move a workpiece 404 (shown in phantom) comprising a plurality of exposure regions 406. For ease of illustration, only some of the exposure regions 406 are labeled 406. The exposure regions 406 are arranged in grid pattern across the workpiece 404, and each of the exposure regions 406 is a region of the workpiece 404 that a photolithography stepper or scanner stopped at and individually exposed to patterned radiation while stepping across the workpiece 404. In some embodiments, the workpiece 404 is as the workpiece 302 is illustrated and/or described in any one of FIGS. 3A-3D. Further, the workpiece stage 402 comprises a plurality of alignment openings 408.

The alignment openings 408 extend through the workpiece stage 402 (i.e., into and out of the page) to enable radiation (e.g., infrared radiation) to pass through the workpiece stage 402. For example, the alignment openings 408 enable radiation from a radiation source (e.g., the radiation source 126 a in FIGS. 1B and 2B) to pass through the workpiece stage 402. As another example, the alignment openings 408 enable an imaging device (e.g., the imaging device 114 a in FIGS. 1A, 1B, 2A, and 2B) to receive and sense radiation through the workpiece stage 402. In some embodiments, cross sections of the alignment openings 408 are as shown in any one of FIGS. 1A, 1B, 2A, and 2B for the upper alignment opening 118 a.

During use of the workpiece stage 402, the workpiece 404 is placed on the workpiece stage 402 so the alignment openings 408 are each aligned to an individual alignment mark on the workpiece. Because the alignment openings 408 enable radiation to pass through the workpiece stage 402, the alignment openings 408 enable alignment marks of the workpiece 404 to be viewed by an imaging device through the workpiece stage 402. This enables alignment of the workpiece 404 to another workpiece (not shown) based on real time feedback on locations of the alignment marks from the imaging device. Further, because the workpiece stage 402 comprises more than one alignment opening, alignment between workpieces may be performed at multiple locations and the workpieces may be rotationally aligned.

In some embodiments, the alignment openings 408 are the same size as, or are larger than, the exposure regions 406 of the workpiece 404 to ensure an imaging device is able to view corresponding alignment marks on the workpiece 404. For example, the alignment openings 408 may have a width W_(ao) that is the same as or greater than a width W_(er) of the exposure regions 406, and/or the alignment openings 408 may have a height H_(ao) that is the same as or greater than a height H_(er) of the exposure regions 406. Further, in some embodiments in which the workpiece stage 402 is used with workpieces with multiple different exposure region sizes, the alignment openings 408 are the same size as, or are larger than, the largest exposure region. In some embodiments, the width W_(ao) of the alignment openings 408 is about 35 millimeters or about 30-40 millimeters. In some embodiments, the height H_(ao) of the alignment openings 408 is about 35 millimeters or about 30-40 millimeters.

With reference to FIG. 4B, a top layout 400B of some alternative embodiments of the workpiece stage 402 of FIG. 4A is provided in which the alignment openings 408 are larger than the exposure regions 406 of the workpiece 404. In other words, the width W_(ao) of the alignment openings 408 greater than the width W_(er) of the exposure regions 406, and/or the height H_(ao) of the alignment openings 408 is greater than the height H_(er) of the exposure regions 406.

With reference to FIG. 5, a cross-sectional view 500 of some embodiments of a three-dimensional integrated circuit (3DIC) chip formed by the bond tool in any one of FIGS. 1A, 1B, 2A, and 2B is provided. The lower workpiece 102 and the upper workpiece 104 comprise corresponding semiconductor substrates 502, corresponding device layers 504, and corresponding interconnect structures 506. The semiconductor substrates 502 may, for example, be bulk monocrystalline silicon substrates, silicon-on-insulator (SOI) substrates, or some other semiconductor substrates.

The interconnect structures 506 are between the semiconductor substrates 502 and contact at a bond interface 508. The interconnect structures 506 comprise corresponding interconnect dielectric layers 510, corresponding wires 512, corresponding vias 514, and corresponding pads 516. For ease of illustration, only some of the wires 512 are labeled 512, only some of the vias 514 are labeled 514, and only some of the pads 516 are labeled 516. The interconnect dielectric layers 510 contact at the bond interface 508, and the pads 516 contact at the bond interface 508. The wires 512 and the vias 514 are alternatingly stacked in the interconnect dielectric layers 510 to define conductive paths electrically coupling the device layers 504 to the pads 516. The wires 512, the vias 514, and the pads 516 are conductive and may, for example, be or comprise copper, aluminum copper, some other metal, some other conductive material, or any combination of the foregoing. The device layers 504 are between, and respectively on, the semiconductor substrates 502. Further, the device layers 504 are separated from the bond interface 508 respectively by the interconnect structures 506. The device layers 504 comprise corresponding semiconductor devices 518. For ease of illustration, only some of the semiconductor devices 518 are labeled 518. The semiconductor devices 518 may, for example, be metal-oxide-semiconductor (MOS) field-effect transistors (FETs), some other MOS devices, insulated-gate FETs, some other semiconductor devices, or any combination of the foregoing.

By using the bond tool in any one of FIGS. 1A, 1B, 2A, and 2B to bond the lower and upper workpieces 102, 104 together, alignment shift S between pads adjoining at the bond interface 508 is low. For example, the alignment shift S may be less than about 30, 35, or 40 nanometers. Because the alignment shift S is low, alignment accuracy between the lower and upper workpieces 102, 104 is high. Further, the pitch P between the pads 516 may, for example, be reduced and/or the width W of the pads 516 may, for example, be reduced. This may, for example, allow the 3DIC chip to be reduced in size.

With reference to FIGS. 6A-6C, cross-sectional schematic diagrams 600A-600C of some embodiments of a bond tool at various stages of a method for workpiece-level alignment and bonding are provided. As seen, the cross-sectional schematic diagrams 600A-600C use embodiments of the bond tool in FIG. 1A. Notwithstanding this, embodiments of the bond tool in FIG. 2A or other embodiments of the bond tool may alternatively be used. Further, while the cross-sectional schematic diagrams 600A-600C shown in FIGS. 6A-6C are described with reference to a method, it will be appreciated that the structures shown in FIGS. 6A-6C are not limited to the method and may stand alone without the method.

As illustrated by the cross-sectional schematic diagram 600A of FIG. 6A, an upper workpiece 104 is placed on an upper workpiece stage 112. In some embodiments, the upper workpiece 104 is placed on an underside of the upper workpiece stage 112. In other embodiments, the upper workpiece 104 is placed on an upper side of the upper workpiece stage 112, and then the upper workpiece stage 112 is flipped vertically to arrive at the orientation shown in FIG. 6A. The upper workpiece 104 comprises an upper alignment mark 108 a and may, for example, be or comprise a semiconductor wafer or some other substrate. The upper alignment mark 108 a may, for example, be cross shaped or some other shape. Further, the upper alignment mark 108 a may, for example, be or comprise copper, some other metal, or some other material that reflects and/or blocks radiation 116 sensed by an imaging device 114 a.

In some embodiments, the upper workpiece stage 112 defines an upper alignment opening 118 a. The upper alignment opening 118 a allows the radiation 116 sensed by the imaging device 114 a to pass through the upper workpiece stage 112 in embodiments in which the radiation 116 would otherwise be unable to do so. As seen hereafter, the radiation 116 may, for example, be or comprise infrared radiation or some other radiation capable of passing through the upper workpiece 104, but not the upper alignment mark 108 a. In embodiments in which the upper workpiece stage 112 defines the upper alignment opening 118 a, the upper workpiece 104 is placed on the upper workpiece stage 112 so the upper alignment mark 108 a is aligned to the upper alignment opening 118 a. This, in turn, allows the upper alignment mark 108 a to be viewed by the imaging device 114 a through the upper workpiece stage 112.

Also illustrated by the cross-sectional schematic diagram 600A of FIG. 6A, a lower workpiece 102 is placed on a lower workpiece stage 110 and aligned to a FOV of the imaging device 114 a. In some embodiments, the aligning is performed at atmospheric pressure or some other pressure. The lower workpiece 102 comprises a lower alignment mark 106 a to facilitate alignment and may, for example, be or comprise a semiconductor wafer or some other substrate. The lower alignment mark 106 a may, for example, be cross shaped or some other shape, and/or may, for example, be the same shape as the upper alignment mark 108 a. Further, the lower alignment mark 106 a may, for example, be or comprise copper, metal, or some other material that reflects and/or blocks the radiation 116 sensed by an imaging device 114 a. The imaging device 114 a may, for example, be configured to sense reflections of the radiation 116 to perform imaging and/or may, for example, comprise a lens 122 a to focus incident radiation on an image sensor 120 a of the imaging device 114 a.

In some embodiments, a process for aligning the lower workpiece 102 to the FOV of the imaging device 114 a comprises laterally moving the lower workpiece 102 with the lower workpiece stage 110 until the lower alignment mark 106 a is in, and aligned to, the FOV of the imaging device 114 a. In some of such embodiments, the FOV and, hence, the imaging device 114 a remain stationary. In other embodiments, a process for aligning the lower workpiece 102 to the FOV of the imaging device 114 a comprises laterally moving the imaging device 114 a and, hence, the FOV until the lower alignment mark 106 a is in, and aligned to, the FOV of the imaging device 114 a. In some of such embodiments, the lower workpiece 102 and hence the lower workpiece stage 110 remain stationary. In yet other embodiments, a process for aligning the lower workpiece 102 to the FOV of the imaging device 114 a comprises using the lens 122 a to optically move and/or manipulate the FOV until the lower alignment mark 106 a is in, and aligned to, the FOV. In some of such embodiments, the imaging device 114 a and the lower workpiece 102 remain stationary.

Regardless of which process is performed to align the lower workpiece 102 to the FOV of the imaging device 114 a, the relative positions of the FOV and the lower alignment mark 106 a are varied based on feedback from the imaging device 114 a. The relative positions may, for example, be varied in a searching pattern until the lower alignment mark 106 a is located within the FOV, as determined from the feedback, and then the relative positions may, for example, be varied into alignment based on the feedback. The searching pattern may, for example, be or comprise a serpentine pattern or some other pattern.

In some embodiments, the upper workpiece stage 112 is moved outside the FOV of the imaging device 114 a before the aligning of the lower workpiece to the FOV of the imaging device 114 a so the upper workpiece stage 112 does not block the imaging device 114 a from viewing the lower alignment mark 106 a. In some embodiments, a radiation source 126 a illuminates the FOV of the imaging device 114 a during the aligning of the lower workpiece 102 to the FOV so the imaging device 114 a can better view the lower alignment mark 106 a.

As illustrated by the cross-sectional view 600B of FIG. 6B, the upper workpiece 104 is aligned to the lower workpiece 102, such that the upper alignment mark 108 a overlies and is aligned to the lower alignment mark 106 a. In some embodiments, the aligning is performed at atmospheric pressure or some other pressure. In some embodiments, a process for aligning the upper workpiece 104 to the lower workpiece 102 comprises moving the upper workpiece 104 laterally with the upper workpiece stage 112 until the upper alignment mark 108 a is in the FOV of the imaging device 114 a and is aligned with the lower alignment mark 106 a. In some embodiments, the lower workpiece stage 110 remains fixed while the upper workpiece stage 112 moves the upper workpiece 104 into the FOV and into alignment. In some embodiments, the FOV and the lens 122 a remain fixed while the upper workpiece stage 112 moves the upper workpiece 104 into the FOV and into alignment. In some embodiments, the lower and upper alignment marks 106 a, 108 a are the same shape and the same size, and alignment occurs when the upper alignment mark 108 a completely covers the lower alignment mark 106 a.

In some embodiments, the moving of the upper workpiece 104 is blind until the upper alignment mark 108 a reaches the FOV of the imaging device 114 a since the imaging device 114 a is unable to view the upper alignment mark 108 a. Therefore, the moving may initially be performed to a location approximately aligned with the lower alignment mark 106 a. The approximately aligned location may, for example, be determined from memory or determined by translation of the known location of the lower alignment mark 106 a to a coordinate system of the upper workpiece stage 112. After the upper alignment mark 108 a reaches the FOV, the moving is based on feedback from the imaging device 114 a. As noted above, the radiation 116 sensed by the imaging device 114 a (e.g., infrared radiation) is capable of passing through the upper workpiece 104, whereby the imaging device 114 a may view the lower alignment mark 106 a through the upper workpiece 104. Therefore, after the upper alignment mark 108 a reaches the FOV, the imaging device 114 a may concurrently view both the lower and upper alignment marks 106 a, 108 a and may hence guide the moving of the upper workpiece 104 based on real-time feedback on the locations of the lower and upper alignment marks 106 a, 108 a. Because of the real-time feedback on the locations of the lower and upper alignment marks 106 a, 108 a, alignment accuracy is high (i.e., alignment error is low) and the number of mechanical movements to perform alignment is low. The low number of mechanical movements, in turn, leads to high throughput and a low cost of ownership.

As illustrated by the cross-sectional view 600C of FIG. 6C, the lower workpiece stage 110 and/or the upper workpiece stage 112 move the lower and upper workpieces 102, 104 into contact. For example, while upper workpiece stage 112 remains fixed, the lower workpiece stage 110 may be move vertically until the lower workpiece 102 is in contact with the upper workpiece 104. In some embodiments, while the lower and upper workpieces 102, 104 are moved into contact, additional alignment is performed between the lower and upper alignment marks 106 a, 108 a. The additional alignment may, for example, comprise laterally moving the lower workpiece 102 and/or laterally moving the upper workpiece 104 so the lower and upper alignment marks 106 a, 108 a are better aligned. As above, the additional alignment is based on real-time feedback on the locations of the lower and upper alignment marks 106 a, 108 a since the imaging device 114 a may view the lower alignment mark 106 a through the upper workpiece 104. This additional alignment further enhances alignment accuracy.

Also illustrated by the cross-sectional view 600C of FIG. 6C, the lower and upper workpieces 102, 104 are bonded together at a bond interface 508 once the lower and upper workpieces 102, 104 are in contact. The bonding may, for example, be performed by fusion bonding, hybrid bonding, some other bonding, or any combination of the foregoing. In some embodiments, an anneal is performed after the bonding to strengthen the bond. Further, in some embodiments, the bonding is performed at atmospheric pressure or some other pressure.

With reference to FIGS. 7A-7C, cross-sectional schematic diagrams 700A-700C of some alternative embodiments of the bond tool of FIGS. 6A-6C at various stages of the method of FIGS. 6A-6C are provided. As seen, the cross-sectional schematic diagrams 700A-700C use embodiments of the bond tool in FIG. 1B. Notwithstanding this, embodiments of the bond tool in FIG. 2B or other embodiments of the bond tool may alternatively be used. Further, while the cross-sectional schematic diagrams 700A-700C shown in FIGS. 7A-7C are described with reference to a method, it will be appreciated that the structures shown in FIGS. 7A-7C are not limited to the method and may stand alone without the method.

The cross-sectional schematic diagrams 700A-700C of FIGS. 7A-7C are respectively as cross-sectional schematic diagrams 600A-600C of FIGS. 6A-6C are described, except that: 1) the radiation source 126 a is below the lower workpiece stage 110; and 2) the lower workpiece stage 110 has a different configuration in some embodiments. Radiation emitted by the radiation source 126 a is incapable of passing through the lower workpiece stage 110 in some embodiments. In such embodiments, the lower workpiece stage 110 defines a lower alignment opening 128 a extending through the lower workpiece stage 110 to allow the emitted radiation to pass through the lower workpiece stage 110 and to illuminate the FOV of the imaging device 114 a. At FIG. 7A, the lower workpiece 102 is placed on the lower workpiece stage 110 so the lower alignment mark 106 a is aligned with the lower alignment opening 128 a.

While FIGS. 6A-6C and 7A-7C illustrated and described alignment using a single alignment location, it is to be appreciated that more alignment locations may be employed for enhanced alignment. For example, with two alignment locations, the lower and upper workpieces 102, 104 may be rotationally aligned.

With reference to FIG. 8, a block diagram 800 of some embodiments of the method of FIGS. 6A-6C and 7A-7C is provided.

At 802, a first alignment mark on a first workpiece is aligned to a FOV of an imaging device based on feedback from the imaging device, where a second workpiece is outside the FOV during the aligning of the first alignment mark. See, for example, FIG. 6A or 7A. During the aligning of the first alignment mark, the relative positions of the first alignment mark and the FOV may, for example, be varied in a searching pattern until the first alignment mark is located within the FOV, as determined from the feedback, and then the relative positions may, for example, be varied into alignment based on the feedback.

At 804, without moving the first workpiece and the FOV, a second alignment mark on the second workpiece is aligned to the first alignment mark based on feedback from the imaging device, where the imaging device views and/or images the first alignment mark through the second workpiece during the aligning of the second alignment mark. See, for example, FIG. 6B or 7B. In some embodiments, the imaging device employs infrared radiation to view the first alignment mark through the second workpiece. During the aligning of the second alignment mark, the relative positions of the second alignment mark and the FOV may, for example, be varied until the second alignment mark is within the FOV, as determined from the feedback, and then the relative positions first and second alignment marks may be varied into alignment based on the feedback.

Because the imaging device may view the first alignment mark through the second workpiece, the first and second alignment marks may be concurrently viewed and/or images. Further, alignment between the first and second alignment marks may be based on real time feedback on the locations of the first and second alignment marks. As a result, alignment accuracy is high (i.e., alignment error is low) and the number of mechanical movements to perform alignment is low. The low number of mechanical movements, in turn, leads to high throughput and a low cost of ownership.

At 806, the first and second workpiece are moved into contact while maintaining alignment of the first and second workpiece. See, for example, FIG. 6C or 7C.

At 808, the first and second workpiece are bonded together. See, for example, FIG. 6C or 7C. The bonding may, for example, be performed by fusion bonding, hybrid bonding, or some other bonding.

While the block diagram 800 of FIG. 8 is illustrated and described herein as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events is not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. Further, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein, and one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases.

With reference to FIG. 9, alignment between the lower and upper alignment marks 106 a, 108 a in FIGS. 6A-6C and 7A-7C is illustrated according to some embodiments. The lower alignment mark 106 a is aligned to the FOV 902 of the imaging device 114 a in FIGS. 6A-6C and 7A-7C. Examples of this alignment process are described at FIGS. 6A and 7A. Further, the upper alignment mark 108 a is moved into alignment with the lower alignment mark 106 a based on feedback from the imaging device 114 a. Examples of this alignment process are described at FIGS. 6B and 7B. Since both the lower and upper alignment marks 106 a, 108 a are in the FOV 902, both the lower and upper alignment marks 106 a, 108 a are visible to the imaging device 114 a. Therefore, the upper alignment mark 108 a is moved based on feedback from the imaging device 114 a that describes the locations of both the lower and upper alignment marks 106 a, 108 a relative to each other. This, in turn, enables high alignment accuracy.

In some embodiments, the present application provides a method for alignment including: aligning a first alignment mark on a first workpiece to a FOV of an imaging device based on feedback from the imaging device, wherein a second workpiece is outside the FOV during the aligning of the first alignment mark; and aligning a second alignment mark on the second workpiece to the first alignment mark based on feedback from the imaging device, wherein the aligning of the second alignment mark is performed without moving the first alignment mark out of the FOV, and wherein the imaging device images the first alignment mark through the second workpiece during the aligning of the second alignment mark. In some embodiments, the imaging device images the first and second alignment marks with infrared radiation. In some embodiments, the first workpiece is on an upper side of a first workpiece stage, wherein the second workpiece is on an under side of a second workpiece stage, and wherein the aligning of the second alignment mark includes controlling the second workpiece stage to laterally move the second alignment mark into alignment with the first alignment mark. In some embodiments, the second workpiece stage defines an opening over the second workpiece, wherein the aligning of the second alignment mark includes imaging the first alignment mark through both the opening and the second workpiece using the imaging device. In some embodiments, the aligning of the first alignment mark includes manipulating the FOV with an optical lens to locate the first alignment mark while the imaging device and the first workpiece are stationary. In some embodiments, the method further includes: after the aligning of the second alignment mark, raising the first workpiece until directly contacting the second workpiece; and bonding the first and second workpieces together. In some embodiments, the aligning of the first alignment mark and the aligning of the second alignment mark are performed at atmospheric pressure. In some embodiments, the method further includes aligning a third alignment mark on the first workpiece to a second FOV of a second imaging device based on feedback from the second imaging device, wherein the aligning of the first alignment mark and the aligning of the third alignment mark are performed simultaneously. In some embodiments, the method further includes aligning a fourth alignment mark on the second workpiece to the third alignment mark based on feedback from the second imaging device, wherein the aligning of the fourth alignment mark is performed simultaneously with the aligning of the second alignment mark, and wherein the second imaging device images the third alignment mark through the second workpiece during the aligning of the fourth alignment mark.

In some embodiments, the present application provides a process tool including: a first workpiece stage configured to support a first workpiece over the first workpiece stage, and further configured to move the first workpiece; a second workpiece stage configured to support a second workpiece under the second workpiece stage, and further configured to move the second workpiece, wherein the second workpiece stage is elevated relative to the first workpiece stage; an imaging device over the second workpiece stage; and a controller configured to control the second workpiece stage to align a second alignment mark on the second workpiece to a first alignment mark on the first workpiece based on feedback from the imaging device, wherein the imaging device is configured to image the first alignment mark through the second workpiece while aligning the second alignment mark to the first alignment mark. In some embodiments, the imaging device is configured to generate the feedback using infrared radiation. In some embodiments, the second workpiece stage defines an opening, and wherein the imaging device is configured to image the first alignment mark through both the second workpiece and the opening while aligning the second alignment mark to the first alignment mark. In some embodiments, the process tool further includes a radiation source over the second workpiece stage and configured to illuminate the first and second workpieces through the opening. In some embodiments, the first workpiece stage defines an opening, wherein the process tool further includes: a radiation source under the first workpiece stage and configured to illuminate the first and second workpieces through the opening. In some embodiments, the imaging device includes an image sensor and an optical lens, wherein the optical lens is configured to focus incident radiation on the image sensor. In some embodiments, the controller is further configured to control the optical lens to manipulate a FOV of the imaging device until the first alignment mark is aligned to the FOV. In some embodiments, the process tool further includes a second imaging device over the second workpiece stage, wherein the controller is further configured to control the second workpiece stage to align a fourth alignment mark on the second workpiece to a third alignment mark on the first workpiece based on feedback from the second imaging device, and wherein the second imaging device is configured to view the third alignment mark through the second workpiece while aligning the fourth alignment mark to the third alignment mark.

In some embodiments, the present application provides a method for wafer-level alignment, the method including: manipulating a FOV of an imaging device using an optical lens until a first alignment mark on a first workpiece is aligned to the FOV, wherein a second workpiece is elevated above the first workpiece and is outside the FOV during the manipulating; and moving the second workpiece laterally until a second alignment mark on the second workpiece is aligned to the first alignment mark based on images from the imaging device, wherein the moving of the second workpiece is performed without moving the first alignment mark out of the FOV, and wherein the imaging device images the first alignment mark through the second workpiece during the moving of the second workpiece. In some embodiments, the method further includes performing infrared imaging during the moving to generate the images, wherein the images concurrently capture the first and second alignment marks. In some embodiments, the moving of the second workpiece is performed by a workpiece stage overlying the second workpiece, wherein the workpiece stage defines an opening over the second workpiece, and wherein the imaging device images the first alignment mark through both the opening and the second workpiece during the moving of the second workpiece.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method for alignment comprising: aligning a first alignment mark on a first workpiece to a field of view (FOV) of an imaging device based on feedback from the imaging device and while a second workpiece is outside the FOV, wherein the aligning of the first alignment mark is performed without moving the imaging device laterally towards the first alignment mark; and aligning a second alignment mark on the second workpiece to the first alignment mark based on feedback from the imaging device and without moving the first alignment mark out of alignment with the FOV, wherein the aligning of the second alignment mark comprises imaging the first alignment mark through the second workpiece using the imaging device.
 2. The method according to claim 1, wherein the imaging device is fixed during the aligning of the first alignment mark.
 3. The method according to claim 1, wherein the FOV is stationary during the aligning of the first alignment mark.
 4. The method according to claim 1, wherein the aligning of the first alignment mark comprises moving the first alignment mark until the first alignment mark is within the FOV.
 5. The method according to claim 1, wherein the second workpiece is supported by a workpiece stage that defines an opening over the second workpiece, wherein the imaging of the first alignment mark is performed through both the second workpiece and the opening using the imaging device, and wherein the method further comprises: scanning or stepping across the first or second workpiece, and individually exposing multiple exposure regions of the first or second workpiece to patterned radiation, using a photolithography tool before the aligning of the first alignment mark, wherein a width of the opening is greater than or equal to that of at least one of the exposure regions.
 6. The method according to claim 1, wherein the first workpiece is supported by a workpiece stage that underlies the first workpiece, wherein the first alignment mark overlies a portion of the workpiece stage during the aligning of the second alignment mark, and wherein the portion is opaque to radiation used by the imaging device for the imaging.
 7. The method according to claim 6, wherein the portion of the workpiece stage is reflective of the radiation.
 8. A method for alignment comprising: performing a first alignment process to align a first alignment mark on a first workpiece to a field of view (FOV) of an imaging device while a second workpiece is outside the FOV; performing a second alignment process to align a second alignment mark on the second workpiece to the first alignment mark based on imaging of the first alignment mark through the second workpiece by the imaging device, wherein the second alignment process is performed without moving the first alignment mark out of alignment with the FOV and achieves a first alignment accuracy between the first and second alignment marks; and moving the first workpiece and/or the second workpiece so the first and second workpieces vertically contact while simultaneously performing a third alignment process, wherein the third alignment process further aligns the first and second alignment marks to each other based on imaging by the imaging device to achieve a second alignment accuracy greater than the first alignment accuracy.
 9. The method according to claim 8, wherein the imaging during the third alignment process simultaneously images both the first and second alignment marks.
 10. The method according to claim 8, wherein an upper workpiece stage overlies and supports the second workpiece, wherein the upper workpiece stage defines an upper opening extending through the upper workpiece stage, and wherein the imaging during the third alignment process is performed through the upper opening.
 11. The method according to claim 10, wherein a lower workpiece stage underlies and supports the first workpiece, wherein the lower workpiece stage defines a lower opening extending through the lower workpiece stage, and wherein the imaging during the third alignment process is performed through both the lower and upper openings.
 12. The method according to claim 10, wherein a lower workpiece stage underlies and supports the first workpiece, wherein the lower workpiece stage has a continuous portion that underlies the first alignment mark during the third alignment process, and wherein the continuous portion is opaque to radiation sensed by the imaging device.
 13. The method according to claim 10, further comprising: stepping across the first or second workpiece while exposing exposure regions of the first or second workpiece to patterned radiation, wherein a width of the upper opening is greater than or equal to that of at least one of the exposure regions.
 14. The method according to claim 8, wherein the first alignment process is performed without moving the FOV laterally towards the first alignment mark.
 15. A process tool comprising: a first workpiece stage configured to support and move a first workpiece over the first workpiece stage, wherein the first workpiece stage has a continuous portion; a second workpiece stage configured to support and move a second workpiece under the second workpiece stage, wherein the second workpiece stage is elevated above the first workpiece stage and defines an opening extending fully through the second workpiece stage; an imaging device over the second workpiece stage and configured to sense radiation, wherein the continuous portion is configured to block the radiation; and a controller configured to control the second workpiece stage to align a second alignment mark in the second workpiece to a first alignment mark in the first workpiece based on feedback from the imaging device, wherein the imaging device is configured to generate the feedback by imaging the first alignment mark through both the second workpiece and the opening while the first alignment mark overlies the continuous portion.
 16. The process tool according to claim 15, wherein the continuous portion of the first workpiece stage extends continuously from a first location to a second location, and wherein the first alignment mark is laterally between and laterally spaced from the first and second locations.
 17. The process tool according to claim 15, wherein the continuous portion is reflective of the radiation.
 18. The process tool according to claim 15, further comprising: a radiation source over the second workpiece stage, wherein the radiation source is configured to emit the radiation towards the continuous portion of the first workpiece stage, and wherein the continuous portion is configured to reflect the radiation towards the imaging device.
 19. The process tool according to claim 15, wherein the controller is configured to control the first and second workpiece stages to align the first alignment mark to a FOV of the imaging device while the second workpiece is outside the FOV and without moving the imaging device towards the first workpiece.
 20. The process tool according to claim 15, wherein the imaging device is stationary. 